I suspect that it's because the seeker become strong contributor in those band, along with interaction with the seeker bulkhead.

with 36 frequencies ranged from VHF down to Ku/Ka band of 36 GHz. Result as follows.

The missile is modeled with dielectric material in seeker and a simple mockup seeker inside it. As seen it has relatively low RCS in typical radar band but surprisingly high in Milimmetric wave range (Ku-Ka)

Both missiles are big, with 50 cm in diameter and some 7.5 meter in length. Naturally i am interested with RCS figure

The estimates are done in HFSS. The missile being modeled is the 48N6DM/E-3 the latest variant of the 48N6 family for S-400.

Further development resulted with longer ranging 48N6 missile which launched in 1984 along with development of much improved S-300PM family.